For the molding of golf balls, especially the molding (final molding) of covers, compression molding and injection molding processes are well known. The compression molding process uses a two-split mold having a plurality of projections corresponding to dimples on the cavity-defining inner wall. After the cover stock is previously shaped into half cups, a core is enclosed in a pair of the cover stock half cups, which is placed in the mold where molding is carried out under heat and pressure. The injection molding process uses a two-split mold having a plurality of gates (typically about 8 equally spaced gates) at the parting line of the mold. After a core is held at the center of the mold cavity to leave a space between the core and the cavity-defining inner wall, a molten thermoplastic resin is injected under pressure into the space through the gates. The mold used in the injection molding process has a venting mechanism including vertically extending cylindrical pins located at the opposite poles of the cavity and holes for receiving the pins to leave gaps, so that air in the space may be discharged through the gaps upon resin injection. The injection mold further has a core supporting mechanism including cylindrical support pins disposed in the mold halves for motion toward and away from the core for supporting the core from vertically opposed directions for placing the core at the center of the cavity, the support pins being received in holes. The support pins are spaced aside from the venting pins.
In the case of compression molding, a ball as molded has an annular burr like Saturn's rings at the parting line or equator of the mold. In the case of injection molding, the resin cures within the gates at the equator to leave gate marks, the resin is squeezed out at the parting line to leave burrs, and the resin penetrates into the gaps between the pins and the holes of the venting mechanism and the support mechanism and cures therein to leave burrs. All of these burrs including burrs at the equator, gate marks, and burrs at the opposite poles and nearby positions must be removed. Trimming is fairly effective for removing burrs, but still leaves burrs 2-1 on a golf ball 1 near one pole as shown in FIG. 3. Since it is difficult to completely remove such burrs by trimming, it is necessary to grind away the burrs 2-1.
Referring to FIGS. 4 and 5, one prior art well-known grinding method which is applied to burrs at the parting line is illustrated. This method is used to remove gate marks resulting from injection molding although it is applicable to the removal of burrs resulting from compression molding. As shown in FIG. 4, a golf ball 1 having gate marks 2-2 along its equator is secured by a pair of holders 3, 3 abutting the opposed poles thereof. A grinding wheel 4 having a working surface 5 with a sufficient width a to cover a region of gate marks 2-2 is disposed such that the working surface 5 will come in contact with the ball 1 at its equator. The abrasive surface 5 is advanced toward the ball 1 while the grinding wheel 4 is rotated in the direction of arrow A' and the ball is rotated in the direction of arrow B' as shown in FIG. 5. The burrs are abraded away under the visual observation of the operator.
Referring to FIGS. 6 to 8, a second prior art grinding method which is applied to burrs near the opposed poles and the parting line is described (see JP-A 63-109880). As shown in the plan view of FIG. 6, a grinding apparatus includes three shafts 6 disposed on the same plane at 120.degree. intervals so that the center axes of all three shafts meet at the center O of a golf ball 1 seated in a golf ball holder (not shown). Each shaft 6 is movable in its axial direction and rotatable about its own axis. The apparatus further includes a mechanism for moving the shafts 6 in their respective axial directions and rotating the shafts about their respective axes. As shown in FIGS. 7 and 8 which are an enlarged cross-sectional view and an end view of the shaft, respectively, each shaft 6 has a working surface 7 which is formed as a concave spherical surface of the same curvature as the surface of the golf ball 1. The working surface 7 is provided with a circular recess 8 for collecting chips (ground pieces of burrs) and radial channels 9 for discharging chips. With this apparatus, grinding is carried out by pushing the shafts against the ball while rotating each shaft in the same direction. Grinding is performed while mutually varying the pressure against the ball and/or rotational speed of each of the three shafts. This causes the ball to turn during grinding, enabling the entire surface of the ball to be uniformly ground.
However, the first method shown in FIGS. 4 and 5 is difficult to precisely control the depth (or stock removal) of grinding since the depth of grinding is determined under the visual observation of the operator. One reason is that the working surface itself is worn and loaded (or dulled). As the working surface becomes worn and loaded, it becomes less abrasive. Then the stock removal of grinding must be adjusted by extending the grinding time or changing the feed rate. The working tool must be replaced when a certain limit is reached, or while the life of the working tool is monitored as a function of the actual use time. It is thus difficult to achieve a consistent stock removal of grinding. Also, since the replacement of the working tool relying on the monitoring of its life generally takes safety allowance, the working tool must be replaced before the expiration of its life.
In the method shown in FIGS. 4 and 5, the operator manages only such factors as the working time and feed rate, and it depends solely on visual observation to determine how much the ball is ground.
As one solution, JP-A 63-99884 proposes a method for grinding a golf ball over its entire surface. The stock removal of grinding from the ball can be confirmed by this method, although it still depends on a trial-and-error process to determine optimum conditions.
The second method shown in FIGS. 6 to 8 not only requires a skilled operator like the first method, but also grinds those regions of the ball's surface that are unnecessary to remove.
In general, a golf ball has on its surface a plurality of dimples which are formed to a depth at a tolerance of within .+-.5 microns. An excessive depth of grinding can destroy the precise surface topography which has been carefully imparted to the dimple-bearing surface of the ball. The resulting decline in the precision of the ball's surface topography, and especially in the precision of the dimple depths, causes the golf ball, when hit, to rise too sharply or to deviate right or left, which is evidence of a deterioration in the flight performance. This problem is inevitable with the second method. Even the first method cannot avoid a certain degree of uneven grinding because of the grinding operation under visual observation, even if a skilled operator is in charge.